IMPROVED CATALYST CONFIGURATION FOR CATALYTIC COMBUSTION SYSTEMS

Abstract: A catalyst bed (30) for a combustor (18) for supporting the catalytic combustion of a gaseous air/fuel (e.g., methane or natural gas) combustion mixture contains an igniter catalyst member (1) upstream of a promoter catalyst member (2). The catalyst members each comprise carrier monoliths, the igniter catalyst member (1) having an igniter catalyst material deposited thereon and the promoter catalyst member (2) having a promoter catalyst material deposited thereon. The igniter catalyst material is distinguished from the promoter catalyst material in one or more of the following ways: the igniter catalyst material may have a) a higher catalytic activity for combustion of the air/fuel mixture, b) a lower catalyst deactivation temperature than the promoter catalyst material, and/or (c) the promoter catalyst regeneration temperature range brackets the upper limit of the regeneration temperature range of the igniter catalyst.

Description: IMPROVED CATALYST CONFIGURATION FOR CATALYTIC COMBTTSTTΩW SYSTffMg Cross-Reference to Related Application This application is a continuation-in-part of co- pending application Serial No. 07/684,409, filed April 12, 1991. BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an apparatus and process for the catalytically supported combustion of gas¬ eous caroonaceous materials, including natural gas and methane. In a more specific aspect, this invention re- lates to an apparatus and process for catalytically sup¬ ported combustion of natural gas or methane using a sup¬ ported palladium oxide catalyst. Description of Related Art Catalytically supported combustion processes have been described in the prior art, e.g., see U.S. Patent 3,928,961 to Pfefferle and U.S. Patents 4,065,917 and 4,019,316. The use of natural gas or methane in catalytic combustion has oeen taught in the art, as has the use of a palladium catalyst to promote such combustion oxidation. See U.S. Patent 3,056,646 to Cohn, wherein the use of pal¬ ladium catalyst ~o promote methane oxidation is disclosed, as is an operable temperature range of 271°C to 900°C (see column 2 , lines 19-25). U.S. Patent 4,154,568 to Kendall et al, dated May 15, 1979 discloses a catalyst bed design comprising a plural¬ ity of carrier monoliths in the flow stream of the air- fuel mixture, wherein the channel size in respective mono¬ liths decreases progressively for monoliths at progres- sively downstream positions, to provide substantially com¬ plete combustion in the catalyst bed (see column 1, lines 47-59). SUMMARY OF THE INVENTION In accordance with the present invention there is provided - πioustor for catalytically promoting thermal combustion : in :nlet combustion gas mixture flowed se¬ quentially r.rεuαh an igniter catalyst zone, a promoter catalyst cone, md then a downstream zone of the combus- tor, the downstream zone providing a homogeneous reaction zone. The ccmpus or comprises the following components. An igniter catalyst member, which may be comprised of one, two or more discrete bodies of igniter carrier, is dis¬ posed in the igniter catalyst zone and comprises an ig¬ niter carrier r.avmg a plurality of gas flow channels ex¬ tending heret rcuαn and defined by channel walls on which an igniter cata yst composition is carried. A promoter catalyst memDer. which may be comprised of one, two or more discrete bodies of promoter carrier, is disposed in the promoter atalyst zone and comprises a promoter car¬ rier having a plurality of gas flow channels extending therethrough an defined by channel walls on which a pro¬ moter catalyst composition is carried. The igniter cata¬ lyst member and the promoter catalyst member together com¬ prise a ombustor catalyst which is characterized by at least one, :r -wo, αr all three of the factors (a), (b) and (o, is roilows: (a) the igniter catalyst member has a higher activity for combustion of the inlet combustion gas mixture - an :oes che promoter catalyst member; (b) the igniter -ataiyst member has a lower catalyst deactivation temperature man does the promoter catalyst member; and (c) the promoter catalyst member has a catalyst regenera¬ tion temperature range which extends from a temperature higher than, to a temperature lower than, the upper limit of the regeneration temperature range of the igniter cata¬ lyst member. in one spect of the present invention, the igniter catalyst member and the promoter catalyst member may each comprise ii crste bodies disposed in proximity to each other, or in abutting contact with each other. A particular aspect of_the present invention provides that the combustor catalyst may be characterized by at least (c) ind rhe regeneration temperature of the promoter catalyst memoer may extend from a temperature at least about 10°C higner. (e.g., about 10 to 400°C higher) to a. temperature at .east about 20°C lower, (e.g., about 20 to 700°C lowerl than the upper limit of the regeneration tem¬ perature range of the igniter catalyst member. One aspect of the present invention provides that the igniter catalyst composition may comprise palladium oxide dispersed on a refractory metal oxide igniter support; the refractory metai oxide igniter support may be selected from the group consisting of one or more of unimpregnated alumina, alumina impregnated with a rare earth metal ox¬ ide, unimpregnated zirconia, zirconia impregnated with a rare earth metal oxide, silica, titania and a co-formed rare earth metal oxide-zirconia. Still another aspect of the present invention pro- vides that the promoter catalyst composition may be sel¬ ected from the group consisting of one of (A) a first pro¬ moter catalyst composition comprising palladium oxide dis¬ persed on a refractory metal oxide promoter support, (B) a second promoter τatalyst comprising a combination of (i) the reaction product of palladium oxide and a metal oxide selected from -.he group consisting of one or more of sama- ria, lantnana inα praseodymia, combined with (ii) a re¬ fractory metal xide binder, and (C) a sequence of (A) and (B) with (Al disposed upstream of (B). The support mater- lal for the t rst promoter catalyst composition may com¬ prise alumina impregnated with a rare earth oxide, and the binder of (ii) may be selected from the group consisting of silica, alumina, alumina stabilized with a rare earth oxide, titania, zirconia and mixtures thereof. As used herein and in the claims, the terms "up¬ stream" and "downstream" refer to the relative placement of element:; censed in the direction of flow of the combus- tion mixture through a catalyst apparatus according to the invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure '. is a schematic plan view of a gas turbine unit utilizing catalytic thermal combustors in accordance with one aspect of the present invention; Figure 2 is a schematic longitudinal cross-sectional view of one of the catalytic thermal combustors of Figure 1 showing four cylindrical catalyst members arranged in sequential, abutting relation therein; Figure 2A is a view taken along line A-A of Figure 2 showing a cross section of catalyst member 1 of Figure 2; Figure 2B s a view, greatly enlarged with respect to Figure 2A, showing in cross section one of the gas flow channels of catalyst member 1; and Figure 3 is a view similar to that of Figure 2 of a catalyst bed according to the present invention, including catalyst members in proximal spaced relation to one anoth- er. DETAILED DESCRIPTION OF THE INVENTION AND SPECIFIC EMBODIMENTS THEREOF Burning ot carbonaceous fuels is associated with for- mation ct air pollutants, among the most troublesome of which are nitrogen oxides (NO χ ). Nitrogen oxides form whenever Jir-supported combustion takes place at open- flame temperatures. One approach to eliminating nitrogen oxides involves catalytic post-treatment to reduce NO χ to nitrogen. A more economical method is to operate the com¬ bustion process catalytically, at a temperature lower than open-flame temperatures, so as to avoid or reduce forma¬ tion of NO ( during combustion. It has long been realized that little or no NO χ is formed in such a catalytic system. Typically, such cata¬ lytic combustion of natural gas or methane utilizes a pre- burner cr thermal combustor which employs flame combustion to preheat combustion air to a temperature of 400°C or higher. Once the catalyst is sufficiently hot to sustain catalysis, the preburner is shut down and all the fuel and air are cirected to the catalyst. Such a catalytic com- bustor, : operated at temperatures below about 1300°C to 1500°C, avoids or at least controls to acceptable levels the NO formation which occurs at the higher temperatures which are characteristic of (non-catalytic) flame combus¬ tion. However, t has generally been regarded as commer- cially infeasibie to conduct such catalytic combustion effectively at high space velocities. Reasons for this lack of commercial attractiveness include the difficulty of economically combusting methane, the principal compo¬ nent of natural as, and the deactivation and instability of the cataiyst compositions employed, especially in the high-temperature end of the catalyst bed where severe high temperatures may be reached. Palladium oxide catalysts are deactivated at temperatures of above about 800°C. Be¬ cause of the susceptibility of the catalyst to such ther- mal deactivation, many catalytic combustor designs are limited i respect to the type and amount of fuel they can combust in order to avoid deleterious high tempera¬ tures. The present invention provides a catalytic combustor design wnicr. .achieves stable combustion of gaseous fuel/- air combust.cn mixtures under controlled conditions to proviαe .j . veis of pollutants, such as carbon monoxide, unburneα hyαrocarbon, and nitrogen oxides in the exhaust gas, and wnicn provides efficient steady state operation of the cmDustor. Generally, the invention relates to an apparatus and process for catalytically supported combus¬ tion of natural gas, methane or other fuels as mentioned below, by utilizing a combustor catalyst having at least two catalyst zones, an igniter catalyst zone and a promo- ter catalyst zone, with the igniter catalyst zone being disposed upstream of the promoter catalyst zone. Both zones comprise least cne catalyst member comprising a carrier coated with a catalyst material. Generally, the catalyst material comprises a platinum group metal, such as palladium oxide, dispersed on a refractory metal oxide support material. The support materials may comprise high surface area alumina, either unimpregnated or impregnated to stabilize the alumina lαamst thermal surface area degradation. Other support materials include unimpregnated zirconia, zirconia impregnated with, e.g., a rare earth metal oxide, a co-formed rare earth metal oxide-zirconia, e.g., ceria— zirconia, such as is described in International Patent Publication 092105861 (PCT/US91/06940) of Engelhard Cor¬ poration, silica or titania. The ataiyst members and the catalyst compositions thereon are jαapted to initiate catalytically-supported combustion at the surfaces thereof of an air/fuel mixture, such as air/natural gas, and to support thermal flame, i.e., homogeneous, combustion of the air/fuel mixture at a point downstream of the catalyst members. By maintaining the homogeneous « flame) reaction zone downstream of the catalyst memoers, the catalyst compositions are not ex¬ posed to deac vating temperatures resulting in decomposi¬ tion of the ictive species, e.g., PdO, and the production of nitrcαen ..cues s further limited. Fuej. rcmcustion advances as the air/fuel mixture flows throuαr. * .".e catalyst members of the combustor and a temper urn :raαιent is therefore established whereby the promoter -one the downstream segment of the catalyst) operates under and is subjected to higher temperatures than the igniter zone (the upstream segment of the cata¬ lyst). For this reason, the present invention provides that the igniter catalyst composition is differentiated from the promoter catalyst composition in at least one of several ways. Generally, according to the present inven- tion, the igniter-promoter catalyst sequence is character¬ ized as exhibition decreasing catalytic activity, increas¬ ing thermal . aoility, or escalating regeneration tempera- ture ranges. It will be appreciated, therefore, that no single catalyst material is.inherently an igniter or pro¬ moter catalyst material; these designations derive from the comparison ;t one or more characteristics of a partic- ular catalyst ateπal to those of another. According to one aspect of the present invention, the catalytic activity of the igniter catalyst composition for initiating combustion of the air/fuel mixture may be high¬ er than that ot the promoter catalyst composition. This differentiation s useful because the igniter catalyst composition must initiate the combustion of a gaseous air/ fuel mixture at a point upstream of, and therefore at a lower temperature than, the promoter catalyst composition. The proper sequencing of any two given catalyst materials according to -.his aspect of the invention can be determin¬ ed by contacting with the catalyst materials a test com¬ bustion mixture comprising a carbonaceous fuel representa¬ tive of the fuel to be burned when the combustor is put to use. For example, a test sample of catalyst material may be disposed on the walls of a suitable honeycomb-type car¬ rier, to provide a test catalyst member, and the test com¬ bustion mixture may be flowed through the test catalyst member. Then, the effluent may be evaluated for its fuel or combustion r αuct content. In one cue." test, the inlet temperature of the test combustion mixture can slowly be increased, and the tem¬ peratures i -men particular levels of fuel conversion are achieved may oe noted, lower temperatures signifying greater activity. Such a test, illustrated below in Ex- ample 3, indicates that the following catalyst materials are given in an order of increasing catalytic activity: 8% PdO dispersed on an unimpregnated (i.e., unstabilized) alumina support; 8% PdO dispersed on a ceria-zirconia sup¬ port; 4% PdO on a ceria-zirconia support or 8% PdO on 10% ceria-impregnated alumina support (the activity of these latter two being similar); 4% PdO on an unimpregnated alu¬ mina support; _.πα ά mixture of 7% La 4 PdO. with 93% alumina as a binder. Therefore, to choose an igniter/promoter catalyst sequence from the foregoing list, any material except the last may be used as the igniter catalyst mater¬ ial and any subsequent material may be used as the prorao- ter material. The activity sequence of other catalyst materials can be determined from activity data provided in the discussion and TABLES that follow or by testing the materials as described herein. It will be appreciated that the invention is not limited to catalyst beds having two catalyst members; additional catalyst members may be employed and are preferably disposed in the catalyst bed in accordance with at least one of the sequencing criteria described herein for the igniter and promoter catalyst members. For example, the promoter catalyst zone may con- tain two catalyst members, each of which comprise a cata¬ lyst material less active than the igniter catalyst mater¬ ial. It should be noted that some variation in activity is known to occur among catalyst materials having the same formulation but having been prepared under different pro¬ cedures. However, given the teachings of this disclosure, one skilled in the art can determine the relative activity of a given catalyst material without undue experimenta¬ tion. In acdit on, the foregoing list of catalyst ateri- als shouid not oe interpreted to indicate that the cata¬ lyst materials according to the present invention must comprise pai aαium; it will be appreciated by those skill¬ ed in the art that catalyst materials may comprise one or more platinum group metals, e.g., one or more of platinum, palladium, rhodium, rutheiniu , osmium and iridium. The choice of catalyst material in a given case may in part be determined by the type of fuel to be burned, the expected operating temperature and by other operating conditions or constraints. As suggested above, natural gas is a common fuel, but the present invention finds utility in processes for combusting other fuels as well, e.g., number 2 fuel oil, ;et fuel, normally liquid hydrocarbon fuels, alco- hols, e.g., methanol, oxygenated hydrocarbons, and even hydrogen, which may be reacted with carbon monoxide.. Alternatively, it may be advantageous for the promot¬ er catalyst Temposition to be more stable than the igniter catalyst composition with respect to exposure to high tem¬ peratures because the promoter catalyst composition is subject to higner operating temperatures. Thermogravimet- ric analysis TGA) may be used to determine the tempera¬ ture at which the catalyst material decomposes to an inactive species, as taught, for example, in U.S. Patent 4,893,465 to Farrauto et al. This Patent, the disclosure of which is incorporated herein by reference, describes experimental data showing that a loss of methane oxidation capability is associated with the decomposition of palla- dium oxice to palladium metal. Generally, TGA involves heating the sample in air at a set rate of temperature in¬ crease and noting when the sample weight begins to change. In the case of palladium oxide catalysts, the onset of de¬ composition is indicated by weight loss as the palladium oxide decomposes to elemental palladium, releasing oxygen. A TGA study of the catalyst materials discussed above with regard to activity suggests the following different se¬ quence o catalyst materials, in order of increasing ther¬ mal stability: i \ PdO on Al 2 0 3 ; 8% PdO on Al,0, ; 4% PdO on ceria-zirconia; 8% PdO on ceria-impregnated alumina; 8% PdO on ceria- irconia; and La. PdO. (tested apart from the alumina mαer.. Decomposition temperatures of these and other materials are given below. While the catalytic activity of the more temperature-resistant catalysts is sometimes lower than that of other catalyst materials, this is not considered to be a detriment in the promoter catalyst zone because the higher temperatures that prevail in this zone can compensate for diminished catalytic ac¬ tivity. Further still, it is known that some catalyst materi¬ als, particularly palladium oxide compounds, can regener¬ ate themselves, i.e., regain catalytic activity after being thermally deactivated by decomposition of the palla¬ dium oxide to palladium metal. Such regeneration can take place spontaneously upon exposure to air within a regener¬ ation temperature range characteristic of the specific catalyst mpioyeα, the maximum regeneration temperature being lower than the deactivation temperature. If the catalyst .z overheated, as may occur due to upset condi¬ tions duπnq operation, it may be regenerated and restored to activity by heat treatment in the regeneration tempera- ture range. Given the temperature gradient that typically exists in combustor beds, it may be advantageous for the promoter catalyst member to have a maximum regeneration temperature wnicn exceeds even the upper limit of the re¬ generation temperature range of the igniter catalyst. In addition, ;mce tne downstream portion of the igniter cat¬ alyst me oor inα the upstream promoter catalyst member are in close proximity, if not abutting one another, it is preferred that the regeneration temperature range of the promoter catalyst extend into the regeneration temperature range of the igniter catalyst material, so that they may regenerate simultaneously. That is, the regeneration tem¬ perature ranαe cf the promoter catalyst may preferably ex¬ tend from a temperature higher than, to a temperature low¬ er than, * r.e .cper limit of the regeneration temperature range of • he .--niter catalyst. Regeπeratisn temperature ranges can be determined using the : Zr :pparatus, by lowering the temperature of decomposeα material and noting when the material begins to exhibit an increase in weight gain signifying the recombi- nation c: :xygen with palladium. The foregoing catalyst materials may be ordered according to the present inven¬ tion in sequence of increasing upper limits for regenera¬ tion as iollows: 4% PdO on unimpregnated alumina; 8% PdO on unimpregnated alumina; 4% PdO on ceria-zirconia; 8% PdO on ceria-zirconia; 8% PdO on ceria-impregnated alumina; and La. PdO. ,without binder). I t »ι.: -e apparent from the foregoing that a se- quence of catalyst materials according to the present in¬ vention may satisfy one or . more of the activity, thermal stability, and regeneration criteria without satisfying another. As indicated above, U.S. Patent 4,893,465, discloses a catalyst somprismg palladium oxide dispersed on an alu¬ mina support ιnd teaches that such a catalyst becomes de¬ activated at aoout 800°C at one atmosphere in air. It also teacnes that the catalyst can be regenerated by low- ering the temperature to a range of from about 530°C to 650°C at atmospheric pressure until the described cataly¬ tic activity is restored. The deactivation and regenera¬ tion of palladium oxide catalysts comprising other support materials s addressed in the U.S. patent application Se- rial No. ?,:"6,907 filed October 16, 1991 and U.S. patent application Serial No. 07/852,371 filed March 13, 1992, both of which are assigned to the assignee of the present application. As discussed in these patent applications, and in particular in application Serial No. 07/776,907, the range of temperatures within which regeneration occurs differs among catalysts having different support materials on which the palladium oxide is dispersed as reflected in the following TABLE I. It is to be noted that all decom¬ position i.nc regeneration temperatures for catalyst spe- cies set rcrtr. in the TABLES and elsewhere herein are based on iecc position or regeneration in air at atmo¬ spheric pressure. It should further be noted that all catalyst species regeneration and decomposition tempera¬ tures reported herein were measured on a Dupont 951 Ther- mal Analyzer ismg heating and cooling rates of 20°C per minute with an air flow of 40 cubic centimeters per minute measured at standard conditions of temperature and pres¬ sure. Those skilled in the art will appreciate that the use of different equipment or different measurement condi- tions could result in somewhat different measured values for such regeneration and decomposition temperatures and that such :ι::srent temperatures may nonetheless be equiv- alent to those described herein once allowance is made for differences in equipment and/or measurement conditions. TABLE I Decomposition and Reformation Temperatures For Palladium Oxide/Palladium on Various Metal Oxide Supports Degrees Centigrade Catalyst'" T,' 1 ' T R (2 I D -T R (3 4% PdO/Al„0, 810 600 210 4% PdO/Ta.O, 4% PdO/TiO. 4% PdO/CeO. 4% PdO/ZrO. ( 1) T D = Decomposition onset temperature of PdO to Pd ' ' T R = Reformation onset temperature of Pd to PdO < 3 > T D -T 3 represents the hysteresis discussed in the patent application. (a | The A \ figure in the first column of the TABLE is the weight percent of palladium as the elemental metal in the catalyst. Thus, f the igniter catalyst composition comprises palladium oxide dispersed on ceria and has a deactivation onset temperature of about 775°C, the promoter catalyst composition may be one having a higher deactivation onset temperature, e.g., palladium oxide dispersed on titania, alumina or tantalum oxide, all of which have decomposition temperatures, i.e., deactivation onset temperatures, higher than 775"C. Arranging the igniter and promoter catalysts accord¬ ing to regeneration temperature will facilitate the in situ regeneration of the catalyst bed because the higher temperatures that exist in the downstream portion of the- promoter catalyst zone might not be low enougn to permit the onset of catalyst regeneration of the igniter cata¬ lyst, but may yet be within the regeneration temperature range of the promoter catalyst. For example, when the ig¬ niter catalyst composition comprises palladium oxide dis- persed en alumina and it is desired to regenerate the cat¬ alysts m situ without shutting down the combustor, it may be useful to employ a promoter catalyst composition com¬ prising pallaαium oxide dispersed on ceria. This arrange¬ ment is suggested by the data in TABLE I, which shows that a PdO/CeO. catalyst begins to regenerate at 730°C, whereas a PdO/Al.O. catalyst must be cooled to about 600°C before it will begin to regenerate. Likewise, the characteris¬ tics of the binary oxide catalyst materials described be¬ low and set forth n TABLES III and IV, would dictate the placement si these compositions in the promoter catalyst zone, particularly in relation to igniter catalysts com¬ prising standard palladium oxide catalyst compositions. Catalyst compositions comprising palladium oxide dis¬ posed on modified alumina supports can have different ac- tivities, deactivation and regeneration temperatures than those comprising an unmodified alumina support, as dis¬ closed in aforesaid application number 07/852,371, and as shown m the loilowing Table II, which appears as Table II in the άforesa c application 07/852,371. ( ι ) "REO" is the rare earth metal content of the samples in moles of the metal per ten grams of fresh alumina. ( 2 ) "Pd" s the palladium metal content of the sample in moles of Pd per ten grams of fresh alumina. ( 3 ) T A = Activity Temperature, the temperature (in de¬ grees Cent αrade) at which combustion of 30% (vol.) of the :H. present in 1% (vol.) CH_. in air mixture takes place at a 1.5 liters per minute flow rate thrc sr. _ :-ιmple of the catalyst weighing 0.06 grams. ( 4 ) T 3β ; = Decomposition Onset Temperature, the tempera¬ ture in degrees Centigrade) at which 80% of the weight loss attributed to PdO decomposition to Pd is attained. ( 5 ) T R = Regeneration Onset Temperature, the temperature (in degrees Centigrade) at which regeneration of the catalyst by oxidation of Pd to PdO commences. ( 6 ) T a -T. represents the hysteresis discussed above ( sic: . The data of TABLE II show that although the inclusion of the lanthanide (rare earth) metal oxides in the alumina generally decreased the activity of the catalyst as indicated by the activity temperature with increasing addition or rare earth oxide, T D80 , the temperature at which 80% ot the weight loss attributed to decomposition of the palladium oxide catalyst is attained, was increas¬ ed by the presence of the rare earth oxide modifier. The catalyst attained by utilizing a lanthanide metal-modi- fied alumina as the metal oxide support is more resistant to high temperatures and therefore would find use in the higher temperature zones of a catalytic combustion cata¬ lyst where its somewhat reduced activity would be more than offset by the increased temperature. it will be noted that different definitions of De- compostion Onset Temperature, T D , as defined in the foot¬ note to TABLE I, and T„„„ as defined in footnote (4) of TABLE II are employed for, respectively, the unmodified (single compound) and modified (more than a single comp- ound) metal oxide supports. This is because whereas the unmodified metal oxide supports such as those listed in TABLE I above exhibit a sharp and definite Decomposition Onset Temperature, the modified metal oxide supports of the type illustrated in TABLE II exhibit decomposition over a broad temperature range, for example, palladium oxide on ce ium-modified alumina supports exhibit decom¬ position : .ln eε ct from about 80 to 131 degrees Centi¬ grade, depenαmg on the palladium oxide loading and the atomic ratio of Ce to Pd. Accordingly, for modified metal oxide supports, the points at which 80% by weight of the total decomposition weight loss occurs was arbi¬ trarily selected as the Decomposition Onset temperature. A catalyst composition may comprise the reaction pro¬ duct of pallaαium oxide with one or more of samaria, lan- thana, neodymia and praseodymia to form binary oxides. The binary oxides of palladium with lanthanum, neodymium and samarium ire discussed in commonly assigned, co-pend- ing U . S . patent appl ication Serial No . 07/684 , 409 filed April 12 , 1991 , the disclosure of which is incorporated by reference herein , and are characterized therein with respect -o act ivity , decomposition and regeneration as shown in TABLE I I I , below, which is a compilation of Table I and an unnumbered Table following Table I in ap¬ plication Ser ial No . 07/684 , 409 . 10 15 ( 1 ) Temperature at which there occurs conversion of 20% 20 of the inlet methane in a 1% methane-in-air mixture flowing through the catalyst at 20 liters per minute at a space velocity of 1,350,000/hour. The solid state reaction products of palladium oxide _ with, respectively, praseodymia, neodymia and lanthana are disc oseα as combustion catalyst materials in common¬ ly assignees, ro-pending U.S. patent application Serial No. 07/684,o31 filed April 12, 1991, now U.S. Patent 5,102,639. The Pr. PdO. material is described as having catalytic activity and a deactivation onset temperature comparable to the other binary oxides, but has a high re¬ generation onset temperature of about 1038°C, as shown in the following TABLE IV, which also appears in U.S. Patent 5,102,639. The subject matter of U.S. Patent 5,102,639 - is incorporated by reference herein. TAELE IV Thermal Properties of - r,PdO 7 ,_Nd 4 PdO 7 and La,PdO ? Primary Active Approximate Species in the Regeneration Reaction Product Td °C Tr °C Weight Gain Pr.PdO. 1220-1310 1038-1000 90% Nd.PdO. 1230-1290 767-730 5% La. PdO. 1300-1350 767-730 5% Td = decomposition temperature, i.e., temperature at which the binary oxide decomposes to the metals. Tr = regeneration temperature, i.e., the temperature at which pallaαiu metal re-oxidizes to palladium oxide or the oi ary oxide. As set forth in U.S. Patent 5,102,639, Pr. PdO. , has vastly superior regenerative properties as compared to the corresponding La and Nd binary oxides. In fact, without wishing to be bound thereof, it is believed that Nd 4 Pd0 7 and La. PdO. , upon being over-temperatured degrade to pro¬ duce palladium metal and regenerate to PdO rather than to the binary oxide. In any case, the binary oxide catalytic material . z typically mixed with a binder material to pre- pare a slurry tor coating onto a carrier monolith. Such binders nci αe for example, silica, alumina, titania and zirconia. .vhen alumina is used, it is preferably impreg¬ nated with a rare earth oxide to inhibit interaction be¬ tween the catalyst material and the binder. The carriers on which the igniter catalyst composi¬ tion and the promoter catalyst composition are carried are typically onoiiths having a plurality of fine gas flow passages extending therethrough, from an inlet to an out¬ let face of the body, to provide a somewhat honeycomb-type structure. The gas flow passages (sometimes referred to as "cells") in the honeycomb structure are substantially parallel ana cefmed by thin walls, and may be of any de- sired cross section such as square, rectangular, triangu¬ lar or hexagonal shape. The number of channels per square inch of face surface, i.e., per cross-sectional square inch (cpsi). nay vary, depending upon the particular ap- plication rcr -men the catalyst bed is to be used. Such honeycomb- ype 'carriers are commerically available having anywhere :rcra mout 9 to 600 or more cpsi. The substrate or carrier -onoiith desirably is porous and may (but need not) be relatively catalytically inert to the combustion reaction as compared to the active layers used in the in¬ vention. The carrier monoliths used in the combustor of the present invention should be refractory in nature, i.e., able to withstand thermal shock caused by the sudden in- crease cr cecr ase m temperature experienced at start-up and shut-down ct the combustor. The monoliths should also have good -r.ermai strength so that they do not develop structural :laws at the operating temperatures of the com¬ bustor, .e., temperatures as high as 1,500°C. Conven- tional corαierite monoliths such as those used to support 3-way catalysts for treating the exhaust gases of automo¬ tive internal combustion engines are generally not consid¬ ered to be suitade in combustors of the present invention because they -an melt or otherwise fail at combustor oper- ating temperatures. More suitable substrates comprise a combination • : cordierite and other oxide materials, e.g., a mixture : —mina, mullite and cordierite. Such sub¬ strates have pnysical properties more suited to combustor operation than conventional ceramic substrates, i.e., bet- ter thermal ctrength and thermal shock resistance, and are commercially available, e.g., from the Dupont Company un¬ der the eslαnation PRD-66. An elemental analysis of this material provided by the Dupont Company describes the material containing 70.4 weight percent Al 2 0 3 , 24.9 weight percent SiO. and 4.2 weight percent MgO. However, another analysis resulted n proportions of about 62.7 - 63.4 weight percent .M.D., 31.2 - 31.3 weight percent SiO, and 5.4 - 5.7 weight percent MgO. A more detailed description of this material may be found in U.S. Patent 5,079,064, the disclosure of which is hereby incorporated herein by reference. For purposes of the present invention, such monoliths ire reterred to herein as "Type I" monolith. The igniter and promoter catalyst members of the pre¬ sent invention .nay be prepared according to known prepara¬ tion techniques, viz., a catalytic material containing the catalyst is applied to a carrier by dipping the carrier into an aqueous slurry of fine particles of the catalytic material, in order to coat the gas flow passage walls. Excess slurry is removed, e.g., by blowing it out of the gas flow passages with compressed air, and the coated structure s dried and then calcined in air at tempera- cures of .iccut l O'C. for about 2 hours to provide an ad¬ herent "washccat" of catalytic material on the walls de¬ fining the gas flow passages. The present invention serves, in another aspect, to alleviate catalyst failures by providing a thermal buffer of separator body disposed in a separator zone situated between the promoter zone where the promoter catalyst mem¬ ber is disposed and the downstream zone where high temper¬ ature homogeneous combustion occurs. The separator body is descπceα more fully in commonly assigned, co-pending U.S. patent ippucat on Serial No. , filed , the disclosure of which is hereby incorporate.! .-.eiein Dy reference. Briefly stated, the separator coαy preferably comprises a monolith similar in configuration to the carriers on which catalyst materials are deposited to form the catalyst members as described herein, i.e., t may take the form of a honeycomb monolith having a plurality of parallel gas flow passages extending therethrough. The separator body is made of a material that can withstand exposure to the high temperatures pro- duced by the homogeneous combustion that occurs in a down¬ stream zone of the combustor. Usually, a catalyst materi- ct r.c 'leposited en the gas flew channels of the separator body although a coating of a refractory metal oxide such as alumina may be coated thereon. Due to its placement between the promoter zone and the downstream zone where homogeneous combustion occurs, the separator cody acts as a thermal shield or insulator to partially insulate the catalyst members from the high temperatures engendered by the homogeneous reaction. The separator body may, but preferably does not, comprise cat¬ alytically active materials, since such materials tend to be vulnerable to deactivation when exposed to the temper¬ atures that the separator body sometimes experiences. Thus, even when the temperature in the downstream zone is excessive, the likelihood of thermal deactivation of the catalyst members and the associated loss in combustion ef- ficiency is diminished. To achieve this effect, the sepa¬ rator body is disposed in close proximity to the promoter catalyst member, i.e., it is either disposed in abutting relation to the catalyst member or is sufficiently close so that the channeled flow of gases through the promoter catalyst member is substantially preserved as channeled flow through the separator body. While Type I monoliths are superior to conventional cordierite monoliths for the reasons discussed above, even Type I monol ths sometimes lose their thermal strength un- der high operating temperature conditions, especially when disposed :n αownstream positions in the combustor. Such weakness . z ceneved to be the result of an attack upon the Type I monolith by high temperature steam produced in the combustion reaction, which leads to deleterious inter- action between the carrier and the active layer thereon. Accordingly, separator bodies used in connection with the present invention comprise a downstream carrier which is stabilized against such degradation. One such support is available from the Minnesota Mining and Manufacturing Co. (3M) under the trade designation "Siconex". These mono¬ lith substrates are described by the manufacturer as being formed fro:.: . series of layers of wcven alumina-boria-sil- ica inorganic fibers. The thus formed monolith is then coated with silicon carbide in a vapor deposition process which is believed to result, after calcination, in a sili¬ ca layer en the surface of the silicon carbide matrix within wnicr. * r. .norganic fibers are disposed. The sili¬ con carbide matrix monoliths have been found to provide superior long--erra thermal strength in relation to the Type I monol ths, and are referred to herein as Type II monoliths. The 3M Company provided an assay of its Type II monolith, which describes the monolith as comprising about 70% silicon carbide and about 30% NEXTEL" 312 ceram¬ ic fibers. The EXTEL 1 " 312 ceramic fibers are described as comprising an alumina-boria-silica material comprising 62 weight percent Al.O. , 14 weight percent B.O, and 24 weight percent iiO.. In a preterred embodiment of the present invention, the igniter catalyst composition comprises palladium oxide dispersed on support material comprising unimpregnated high surface area alumina and the promoter catalyst com- position preterably comprises palladium oxide dispersed on high surtace area alumina which is stabilized against thermal degradation in a conventional manner known in the art, e.g., by the impregnation into the alumina of a sta¬ bilizing exice, preferably a rare earth metal oxide in particular, ceria. Preferably, the igniter catalyst mem¬ ber and the promoter catalyst member comprise Type I mono¬ liths, .- " urtr. r. .t is preferred to dispose a separator body αisposeα jownstream of the catalyst members to ther¬ mally shield the catalyst members from the heat produced n the αownst eam zone, and the separator body preferably comprises a Type II monolith as described below. PreferaDly, the igniter catalyst member and the pro¬ moter cataiyst member and, if present, the separator body, are discrete oodles within the combustor. In other words, the igniter catalyst member will preferably comprise the igniter catalyst composition disposed on the igniter car¬ rier .-.no r.e prc oter catalyst member will likewise com- prise the promoter catalyst composition on a separate pro¬ moter carrier. Then, the igniter catalyst member and the promoter cataiyst member may be disposed within the com¬ bustor in a iacent, preferably in abutting, upstream/down- stream reiat cn to one another. The catalyst members, thus disposed .n proximity to each other, are preferably disposed with * neιr respective gas flow channels in mutual alignment. Thus, the flow of combustion gases through the igniter catalyst member will be channeled into the promot- er catalyst emoer. If the two catalyst members are not abutting, they should be in close proximity, whereby the channeled gas flow is maintained between them. Alterna¬ tively, the -cniter catalyst member and the promoter cata¬ lyst member .-.ay oe formed on a single, integral monolith by applyir.'.i u -eating of the igniter catalyst composition on one end o t the monolith and a coating of the promoter catalyst composition on the other end of the monolith. The separator Dody, which also comprises a refractory body having a plurality of gas flow channels extending there- through, may likewise be part of a single monolith with a promoter cataiyst member, with catalyst material being deposited on enly one end of the monolith to provide the catalyst memoer, the other end providing the separator body. ~pticna * ly, the separator body monolith may have a coating thereon of a refractory inorganic oxide such as alumina. Ret'-.::'::-.-: .ow to Figure 1, there is shown m schemat¬ ic plan v ew ι τa≤ turbine 10 comprising a starter engine 12 connected y an engine shaft 14 to an air compressor 16, whicn . z provided with inlet air, via air inlet lines indicated by arrows a, which is compressed by compressor 16 and discharged v a lines a' into combustion gas inlet lines c which are also supplied with a pressurized gaseous fuel, such as natural gas or methane, via gas inlet lines indicated by arrows f. The air and fuel combine to form a combustion mixture which is introduced via lines c into a plurality :f :a-alytic thermal combustors 18, two of which are illustrated n Figure 1 although it will be appreci¬ ated that any suitable number may be employed. For exam¬ ple, eight such combustors 18 may be utilized with their outlets disposed equiradially about the inlet to the tur- bine. Eacn catalytic thermal combustor 18 is provided with an associated outlet duct 20 connected in gas flow communication h a turbine 22 which may comprise a mul¬ ti-staged turbine as well known to those skilled in the art. Turbine 22 is drivingly connected to a load coupling shaft 24 to connect the turbine output to a suitable de¬ vice, for example, an electric generator. The expended combustion products are exhausted as shown by arrow e via exhaust stac 26 :or discharge to the atmosphere or for further use or processing. Figure 2 snows a schematic cross-sectional view of a typical catalytic thermal combustor 18 comprising a can- nister 19 having an inlet section 28, a catalyst bed or section 30 containing catalyst members 1, 2, and 3, and a downstream zone 31 including a separator body 4 and a ho- mogeneous flame section 32. The three catalyst members 1, 2, and 3, and separator body 4 are arranged in sequential, abutting contact. That is, catalyst members 1 and 2 are positioned in iace-to-face abutting contact, as are cata¬ lyst members 2 and 3. Catalyst member 3 and separator body 4 are aiso in abutting contact. In other embodi¬ ments, other arrangements may be used. For example, 1 and 2 may comprise catalyst members and 3 and 4 may comprise separator bodies. One or more of the catalyst members, e.g., catalyst member 1, comprises an igniter catalyst as described below and one or more catalyst members, e.g., catalyst member 2, located downstream of the igniter cata¬ lyst member comprises a promoter catalyst as described below. Generally, the catalyst members, e.g., 1, 2, and 3, and separator body 4 comprise a refractory substrate mate¬ rial formed into what is sometimes referred to as a mono¬ lithic rr :-.cnevcomD substrate or carrier. The carrier is a substantially cylindrical body (see Figure 2A) having opposite end faces between which extend a plurality of generally parallel, fine gas flow passages. Figure 2A shows a typical catalyst member end face la of catalyst member 1, schematically showing a plurality of fine, par¬ allel gas flow passages extending longitudinally through catalyst member 1 to permit gas flow through catalyst mem¬ ber 1. This construction is typical of all the catalyst members 1 through 3 inclusively. The gas flow passages are defined by walls on which are disposed a coating (of¬ ten referred to as a "washcoat") of an active material suitable to catalyze the oxidation of a gaseous fuel such as natural gas or methane. Figure 3 shows a similar con¬ figuration, except that segments 3 and 4, instead of being m mutual αouttmg relation, are in proximal relation to one another. Figure 2B shows an enlarged view corresponding to Figure 2A in which a typical gas flow passage 34 is shown in cross-sectional view as being defined by four gas flow passage walls 34a on which is coated a catalytic material washcoat 36. The cross-sectional configuration of gas flow passage 34 illustrated in Figure 2B is rectangular but it will be appreciated that any suitable cross-sec¬ tional cent lauration may be employed such as square, poly- gonal, e.g., triangular, or circular. Further, the gas flow passaαes may have a configuration attained by alter¬ nating .JVC:: C: flat and wave-form plates made of a suitable retractory material, as is well known to those skilled in the art. Example 1 To illustrate the effectiveness of 2La 2 O..PdO for methane combustion, its performance was compared to a blank alumina washcoated honeycomb. Two catalyst bodies measuring 1 inch in diameter by 8 inches in length were prepared. The first 6 inches of each body was the same and comprised '..5 g/in : PdO on alumina catalyst, coated onto a 64 cell per square inch alumina-silica-magnesia fiber honeycomb type support. The last 2 inches of . a comparative sample was provided with a plain alumina wash¬ coat, wniie the last two inches of an exemplary catalyst composition as provided with about 1.5 g/in of a wash¬ coat containing about 7 weight percent of the catalytic species 2La. .?d0, balance (93 weight percent) alumina as a binder. The a.Pd0 7 was prepared by a method which comprises mixing an oxide of lanthanum with palladium oxide in selected weight ratios. The mixture is mechan¬ ically ground to a size range of about 50 to 100 micron diameter particles. The grinding is followed by calcina¬ tion in air, for example, to provide a reaction mixture containinq the cmary oxide of palladium and lanthanum. Preferably, the ^ anthana and palladium oxide starting ma¬ terials are mixed in stoichiometric proportions to produce the desired compound. Thus, the molar ratio of the lan- thana to PdO in the reaction mixture may be 2: 1, 1:1 or 1:2. Although it is not necessary to use the starting ma- terials in the molar ratios of the desired binary oxide product, the use of such stoichiometric proportions has been found to be advantageous as described in aforesaid U.S. patent application Serial No. 07/684,409. The conversion of a combustion mixture comprising 4 volume percent -ethane in air was measured in a pilot re¬ actor under three atmospheres pressure and 50 ft per sec¬ ond linear .-ei city of gas flow measured at inlet tempera¬ ture ana pressure. The comparative reactor containing the alumina Dlanκ showed a temperature increase of 200°C across the reactor with an inlet gas temperature of 475°C. The exemplary reactor containing 2La 2 0 3 .PdO produced a temperature increase of 350°C across the reactor under the same inlet gas conditions. The 150°C greater increase in temperature across the exemplary reactor as compared to the comparative reactor is due to improved combustion in¬ duced by the 2La..O, .PdO catalyst. The exemplary catalyst, comprising c.-.iy 2Ξ " of the length of the bed, induced a 75% improvement in overall efficiency of combustion as compared to a reactor in which 25% of the bed was compris¬ ed of a blanκ alumina catalyst. The exemplary catalyst also supported -embustion of methane at close to 100% ef- ficiency i h in ♦;£fluent containing less than 5 ppm un- buraed hyαrocarnons and less than 2 ppm nitrogen oxides, while operating under catalytically-supported thermal com¬ bustion conditions of the type described below with refer¬ ence to Pfetferie U.S. Patent 3,928,961. The relatively modest temperature increment across the exemplary reactor and the exceedingly low level of hydrocarbons in the ef¬ fluent clearly indicates that desired catalytically sup¬ ported thermal combustion was attained, i.e., hydrocarbons were combusteα uownstream of the catalyst. Example 2 A. A catalytic combustion system was prepared as follows with two sections of catalyst, a first or upstream section positioned to be first contacted by the flowing gaseous comoustion mixture stream, and a second or down¬ stream section oemg last contacted by the flowing gase¬ ous stream. B. The tirst section was a catalyst body provided by a cordieπtc --.cneycomb having 400 cells per square inch of end face mα cemg 1 inch (2.54 cm) in diameter, and 4 inches '.?.2 -mi long and coated with 1.1 g/in 3 of alumina washcoat ;:;iι.:;:nq 8% PdO by weight of the dried, cal¬ cined washcoat. C. The second section was a catalyst body provided by a honeyccmo made of silica-alumina-magnesia fibers and of the same overall dimensions as used for the first sec¬ tion, but it 6 cells per square inch, coated with a washcoat ct aiumma mixed with 2La 2 0 3 .PdO particles and having a content of 1% by weight PdO equivalent contained in the binary oxide. D. The combustion of methane was measured under the following cr.citicr.s. A gaseous stream containing about 4% by volume methane in air was flowed at a velocity of 50 feet per second and at three atmospheres pressure in se¬ quence throuqh the first and second catalyst bodies. At an inlet temperature of 480°C, combustion of methane was complete, unnurneα hydrocarbons and oxides of nitrogen emissions were below 2 parts per million ("ppm"). The results of Examples 1 and 2 show that a catalyst composition in accordance with the invention may be uti- lized as a downstream promoter catalyst in combination with an upstream igniter catalyst comprised of a more active catalyst such as PdO. It is seen that the upstream or inlet portion of the catalytic system operates at a lower temperature than does the downstream portion. Con- sequentiy, -. more active catalyst but one which decomposes at relatively low temperatures (such as a PdO catalyst) may be used in the cooler, upstream igniter zone whereas a catalyst, such as a binary oxide catalyst described above, which is comparatively more resistant to high temperature may be used in the downstream or higher temperature pro¬ moter zone cf the catalyst. Since the promoter zone cata¬ lyst is contacted by the higher temperature gases emerging from the igniter zone, the lower activity of the high tem¬ perature resistant catalyst is nonetheless adequate to catalyze comoustion of the fuel at the higher temperatures involved. Example 3 A series of catalyst members were tested to determine their relative catalytic activities, thermal stability and regeneration temperature ranges so that they could be dis¬ posed in the proper sequence according to the present in¬ vention. The catalyst members all comprised Type I mono¬ liths coated with various catalyst materials at a loading of 1.5 g/in . The configurations of the various test mem- bers, which were designated Zl through Z6, are summarized below m TABLE IIIA. The cataiyst materials for test members Zl through Z5 were prepared by impregnating particles of a refractory inorganic oxide support material with a palladium salt so¬ lution, and then calcining the impregnated particles to leave palladium oxide deposited thereon. The alumina used as a support material for catalyst members Zl and Z2 was unimpregnated high surface area alumina. The ceria-zir¬ conia support material used for test members Z3 and Z4 was a co-precipitated ceria-zirconia material comprising about 12% by weiσnt ceria and about 88% by weight zirconia. The co-prec p tαt on results in the ceria being dispersed uni¬ formly thrcuσnout the zirconia matrix. This differs from the res .-. ittαmed by impregnation techniques commonly used in the art, which would have left ceria only at the surfaces or the zirconia particles if zirconia particles were impregnated with a solution of a certain salt. The Z5 catalyst was prepared by co-impregnating alumina par¬ ticles w th a solution of cerium nitrate and palladium nitrate, and then calcining the impregnated alumina parti¬ cles to produce a material comprising, by weight, 8% pal- ladium as palladium oxide and 10% ceria. The La 4 Pd0 7 of test member 36 was prepared as described above in Example 1, and was αdm ::ed with alumina which acts as a binder. The Z6 catalyst material comprised, by weight, 7% of the La 4 Pd0. and 93% alumina. _Each of the foregoing catalyst materials were formed into a slurry which was deposited onto a Type I onoiith by immersing the monolith into the slurry, αryir.q and calcining the coated monolith, to pro¬ vide a coating of 1.5 grams catalyst material per cubic inch. After the carriers were coated, test members Zl through 24 were calcined at 500°C for 2 hours, and then aged at 50'C for 18 hours. Member Z5 was calcined at 500°C for 2 hours and then aged at 850°C for 18 hours. Test member 36 was calcined at 500°C for 2 hours and aged at 1100 " C for 13 hours. Following calcining and aging, the activity of each of the test memoers was evaluated by flowing a test com¬ bustion fixture comprising 1% methane in air at one at¬ mosphere pressure and at a velocity of 20 ft/sec over a catalyst test core one-half inch in length and three quar¬ ters of an inch in diameter. The temperature of the inlet stream was raised, and the temperatures at which various degrees of conversion of the methane were attained were noted. If, at a temperature of 700°C, the desired rate of conversion was not achieved, the actual conversion rate attamec it that temperature is stated in parenthesis. The results cire set forth below in TABLE IIIB. TABLE IIIB Temp.°C @ %CH. Conversion The data of TABLE IIIB indicate that any of the cata¬ lyst members Zl - Z6 may be selected in a prescribed se¬ quence as an igniter or promoter catalyst in accordance with the present invention. In the following sequence, a first listed catalyst may serve as an igniter catalyst for any subsequently-listed catalyst: Z2, Z4 or Z5 (Z4 and Z5 are approximately equivalent), Z3, Zl, Z6. The catalysts are listed in descending order of relative igniter cata¬ lyst capability. Thus, in accordance with the present in- vention, catalyst Z2 could serve as an igniter catalyst with any of the subsequently listed catalysts serving as a promoter catalyst. Similarly, catalysts Z4 or Z5 could serve as the igniter catalyst with any of catalysts Z3, Zl or Z6 serving as the promoter catalyst, and so on. In addition, the catalyst material of catalyst mem¬ bers Zl thrcuαn 26 were evaluated to determine their de¬ composition and regeneration temperature ranges. The re¬ sults are set forth below in TABLE IIIC. TABLE IIIC The data of TABLE IIIC indicate that catalyst members Zl - Z6 may be chosen as igniter and promter catalysts from the following sequence of increasing thermal stabil¬ ity, i.e., escalating minimum degradation temperatures: Zl, Z2, 23 or 25, Z4 , Z6. Alternatively, they may be chosen from the following sequence according to escalating regenerating temperature ranges: Zl, Z2, Z3, Z4, Z5, Z6. Example 4 Catalyst beds designated D, S and T, each .comprising different catalyst sequences falling within the present invention were prepared and tested. The beds each com- prised four segments, each comprising a Type I monolith having 64 cells per cross-sectional square inch (cpsi). Segments 1 , 3 and 4 were all 1.5 inches in length whereas segment 2 was 1 inch in length in each bed. Various mate¬ rials indicated below in TABLE IVA were coated onto the carrier monoliths by immersing the monoliths in slurries containing the catalyst materials, drying and calcining the immersed carriers so that the loading on the monoliths was 1.5 g/in in each case. The catalyst materials were prepared or described in the foregoing Examples. The ac- tivity, thermal degradation and regeneration temperatures of the catalyst materials used are set forth in Example 3. The configurations of the beds of this example are sum¬ marized in the following TABLE IVA. It will be noted that in each of the three cases illustrated in TABLE IVA, seg- ments 3 and 4, wnich contain no palladium catalysts, are coated with alumina only, and serve as separator bodies which thermally shield catalyzed segments 1 and 2 from the homogeneous reaction zone. T2 1" 8 wt. % Pd; 10% ceria/ alumina T3 1.5" alumina T4 1.5" alumina Bed 2 sequence Z2, Z5) met all the criteria discuss¬ ed above reαaremg the sequence of catalyst materials, but bed S (sequence 21, Z5) only met two of the criteria, since the -aterial on segment S2 is more active than that of segment 21. s reflected in Example III above. Bed T (sequence 24, 25) met only the criterion relating to re¬ generation temperature, since segment T2 begins to degrade at a lower temperature than does Tl and since the relative activities are not distinguishable. Beds 3, 5 and T were each placed in a combustor to show their efficacy in initiating the combustion of an air-fuel mixture comprising about 4% methane in air at 3 atmospheres pressure. Each bed initiated downstream homo¬ geneous combustion at the inlet temperature, gas stream velocity, αno uei concentration given in TABLE IVB below. TABLE IVB Ignition Conditions Cat. Inlet Init. Vel. Fuel Cone. Extinction Bed Tem .. °r, , ft/s) Vol. (%) Temp. (°C)/Fuel D 487—50 50 4.1-3.75 530 60 4.0 496 / 4.0 485-495 30 4.0 465-485 / 4.0 S 490-530 50 ca. 4.0 10 T 510-525 60 4.0 512 50 4.0 550 30 4.0 Example 5 The cata vst ced designated as M was prepared in l c whicn igniter cataiyst member Ml comprised a catalyst washcoat comprising 4% palladium oxide on a co-formed rare earth metal oxide (i.e., ceria) - zirconia support materi¬ al, which was coated on a Type I substrate. The prepara¬ tion of the co- ormed ceria-zirconia material is described 20 above n Example 2. Promoter catalyst member M2 and sep¬ arator body members M3 and M4 were prepared in the same manner as catalyst members D2 (8% Pd; 10% ceria/alumina) , D3 and 4 .both alumina on a Type I substrate), respec¬ tively. The cent guration of bed M is summarized below in 25 TABLE VA. Bed M met the invention criteria relating to decompo¬ sition and regeneration temperatures but not activity, probably because of the greater palladium oxide content on segment M2 compared to segment Ml. The bed was tested and was found to .gnite a combustion mixture comprising 3.8 to 4.0 percent methane in air at an inlet velocity of 38 to 40 ft/sec at a qas inlet temperature of 407 to 414°C and a pressure ot . atmospheres. The exhaust gases contained no detectable carbon monoxide. Example 6 Three beds designated C, E and F were prepared to demonstrate the combination of catalyst sequences in ac¬ cordance with the present invention with the preferred separator cccy comprising a Type II substrate. TABLE VIA describes the configurations of the three catalyst beds. Example 6 in TABLE VIA and VIB shows the efficacy of catalyst sequences in accordance with the present inven¬ tion for catalyzing the combustion of a gaseous combustion mixture. In addition. Example 6 and its associated TABLES demonstrate the successful use of catalyst substrates in which all the substrates are Type I (catalyst bed C) in which the catalyst substrates are Type I and the separator bodies are Tvpe II substrates (catalyst bed E) and in which the r rst two catalyst substrates are Type I, and the third catalvst substrate and the separator body are made of i Type :i substrate (catalyst bed F). Generally, the catalysts ot the present invention disposed on Type I substrates nave generated better activity and performance than the catalysts dispersed on Type II substrates. On the other nand, catalysts of the present invention dis¬ persed on the Type II have demonstrated somewhat better durability and resistance to hydrothermal stresses. Co- pending patent application Serial No. , filed aescnbes and claims specific sequences of Type I and Type II substrates disposed within a single jo oustci -r. :rc r to attain certain cascribed advantages. Generally, by utilizing the Type I substrates in the up¬ stream portion of the combustor bed and the Type II sub¬ strates in a downstream portion of the combustor bed, en¬ hanced activity s attained for the upstream segment(s) and enhanced curaDility is attained by the downstream segment( s ) . All the Type I substrates in catalyst beds C, E and F had 64 cells per square inch, and all the Type II sub¬ strates had 60 cells per square inch. The washcoat load¬ ings on the catalyst members of beds C, E and F was 1.5 g/in' The bed configurations may be summarized as fol- lows: TABLE VIA C3 1.5" 7 wt.% 2La 2 0,.PdO with 97% alumina binder C4 1.5' alumina Catal st Bed E TABLE VIA (CONTINUED) Catalyst Bed F F3 II 1.5" 7 wt.% 2La 2 0..PdO with 97% alumina binder F4 II 1.5" alumina The efficacy of catalyst beds C, E and F were tested by placing them in a combustor to determine their respec¬ tive initiation temperatures for a 4% methane in air com¬ bustion mixture at a pressure of 3 atmospheres. Two eval- uations were performed for beds C and E, and three evalua¬ tions were made for catalyst F. The results are set forth below in TABLE VIB. Whiie the invention has been described with reference to particular emoooiments thereof, it will be appreciated that numerous variations to the described embodiments will be within the scope of the appended claims.

Claims: THE CLAIMS What is claimed .s: 1. Λ cc oustor for catalytically promoting thermal combustion ot an inlet combustion gas mixture flowed se¬ quentially through an igniter catalyst zone, a promoter catalyst zone, and then a downstream zone of the combus¬ tor, the downstream zone providing a homogeneous reaction zone, and the combustor comprising: (i) an igniter cata¬ lyst member disposed in the igniter catalyst zone and com¬ prising an igniter carrier having a plurality of gas flow channels extending therethrough and defined by channel walls on whicn n igniter catalyst composition s carried; and (ii) α prcmoter catalyst member disposed in the promo¬ ter catalyst cone and comprising a promoter carrier having a plurality ot gas flow channels extending therethrough and defined by channel walls on which a promoter catalyst composition is carried, the igniter catalyst member and the promoter catalyst member together comprising a combus¬ tor catalyst -inch is characterized by at least one of the following: ai the igniter catalyst member has a higher activity lor combustion of the inlet combustion gas mix¬ ture than αoes the promoter catalyst member, (b) the ig¬ niter cataiyεc -.emoer has a lower catalyst deactivation temperature ~han αoes the promoter catalyst member, and (c) the promoter catalyst member has a catalyst regenera¬ tion temperature range which extends from a temperature higher than, to α temperature lower than, the upper limit of the regeneration temperature range of the igniter cata¬ lyst member. 2. The combustor of claim 1 wherein the combustor catalyst s characterized by at least two of (a), (b) and (c) . 3 . The comoustor of claim 1 wherein the combustor catalyst s characterized by each of (a), (b) and (c). 4. The combustor of claim 1, claim 2 or claim 3 wherein the :αnιter catalyst member and the promoter cata¬ lyst member •"•ach comprises a discrete body and wherein the members are disposed in proximity to each other. 5. The comoustor of claim 1, claim 2 or claim 3 wherein the igniter catalyst member and the promoter cata¬ lyst member each comprises a discrete body and wherein the members are disposed in mutual abutting contact. 6. The comoustor of claim 1 wherein the combustor catalyst is characterized by at least (c) and the regener¬ ation temperature of the promoter catalyst member extends from a temperature at least about 10°C higher, to a tem¬ perature at least about 20°C lower, than the upper limit of the regeneration temperature range of the igniter cata¬ lyst member. 7. The combustor of claim 6 wherein the regeneration temperature of the promoter catalyst member extends from a temperature wnich is at least about 10°C to 400°C higher, to a temperature wnich is at least about 20°C to 700°C lower, than the upper limit of the regeneration tempera¬ ture ranαe f the igniter catalyst member. 8. The comoustor of any one of claims 1, 2 or 3 wherein the igniter catalyst composition comprises palla¬ dium oxide dispersed on a refractory metal oxide support. 9. The combustor of claim 8 wherein the refractory metal oxide igniter support is selected from the group consisting of one or more of unimpregnated alumina, alu¬ mina impregnated with a rare earth metal oxide, unimpreg¬ nated zirconia, zirconia impregnated with a rare earth metai oxiαe, silica, titania and a co-formed rare earth etal oxide-zirconia. 10. The combustor of claim 9 wherein the refractory metal oxide support comprises unimpregnated alumina. 11. The combustor of claim 8 wherein the igniter catalyst eπuoer comprises two or more discrete bodies of igniter carrier. 12. The combustor of claim 8 wherein the promoter catalyst composition is selected from the group consisting of one of ;A) a first promoter catalyst composition com¬ prising palladium oxide dispersed on a refractory metal oxide promoter support, (B) a second promoter catalyst comprising α comomation of (i) the reaction product of palladium oxide and a metal oxide selected from the group consisting of one or more of samaria, lanthana and praseo- dymia, combined with (ii) a refractory metal oxide binder, and (C) a sequence of (A) and (B) with (A) disposed up¬ stream of ( B) . 13. The combustor of claim 12 wherein the first re¬ fractory metal oxide promoter support comprises alumina impregnateα with α rare earth metal oxide and the refrac¬ tory metai cxioe binder of (ii) is selected from the group consisting of cne or more of silica, alumina, alumina sta¬ bilized ιtr. .i rare earth metal oxide, titania and zirconia. 14. The combustor of claim 12 wherein the promoter catalyst member is comprised of two or more discrete bod¬ ies of the promoter carrier. 15. The combustor of claim 12 wherein the promoter catalyst composition comprises the first promoter catalyst composition. 16. The combustor of claim 15 wherein the first re¬ fractory metal oxide prompter support comprises alumina impregnated with a rare earth metal oxide. 17. The comoustor of claim 16 wherein the first re¬ fractory metal oxide promoter support comprises alumina impregnated w th with a rare earth metal oxide selected from the group consisting of one or more of lanthana, ceria and praseodymia. 18. The combustor of claim 12 wherein the igniter composition comprises palladium oxide dispersed on a re¬ fractory metai oxide igniter support. 19. The comoustor of claim 18 wherein the promoter catalyst me oer comprises the first promoter catalyst. 20. The combustor of claim 19 wherein the promoter catalyst member is comprised of two or more discrete bod¬ ies of the promoter carrier. 21. The combustor of claim 18 wherein the refractory metal oxide igniter support comprises unimpregnated alumi¬ na, the refractory metal oxide promoter support comprises alumina impregnated with a rare earth metal oxide, and the refractory metal oxide binder, if present, is selected from the jroup consisting of one or more of silica, alumi¬ na, alumina stabilized with a rare earth metal oxide, ti¬ tania and zirconia.

Inventors: FARRAUTO ROBERT J, FEELEY JENNIFER S, KENNELLY TERESA, LUI YIU KWAN, SIMONE DIANNE O, FARRAUTO, ROBERT, J., FEELEY, JENNIFER, S., KENNELLY, TERESA, LUI, YIU, KWAN, SIMONE, DIANNE, O.
Applicants: ENGELHARD CORP, ENGELHARD CORPORATION
Application year: 1994
Main classification: F23C11
IPC codes: B01J23, F23C13, F23R3